The present invention pertains to strapping machines. More particularly, the present invention pertains to an improved winder for a strapping machine that uses the strap tension to secure the strap in the winder.
Strapping machines are in widespread use for applying a strap, such as a plastic strap, in a tensioned loop around a load. A typical strapping machine includes a strap chute for guiding the strap around the load, a strapping head through which the leading end of the strap is fed, and a strap dispenser to dispense a desired length of strap from a coil of strap material.
The strapping head carries out a number of functions. It advances the strap along the chute around the load until the leading end returns to the strapping head and retracts or rewinds the strap from the chute to produce tension in the strap around the load. The strapping head typically includes an assembly for securing the strap in the tensioned loop around the load such as by welding the strap to itself at its overlapping portions.
A typical strapping head includes a pair of advancing rollers for advancing the strap through the strapping head and a pair of retraction rollers for retracting the strap to, for example, take-up the strap. The head also includes a winder or tensioner that rewinds or takes up the strap after it is positioned around the load so as to apply a tension in the strap. In one known configuration, the winder includes a split-type rotating element that has a channel or slot formed therethrough to essentially define split halves of the winder. The split halves are fixed relative to one another and the strap traverses through the slot between the halves. Upon an appropriate signal, the winder is actuated and rotates to tension the strap.
In this arrangement, the strap may not be in tension until it passes over itself around the winder, thus creating sufficient friction to prevent the strap from slipping through the winder slot. It has been observed that often, the winder must rotate in excess of 360 degrees, and with some types of readily compressible loads, it must rotate more than 720 degrees to provide sufficient friction to begin tensioning and to provide the appropriate tension on the strap. This can be problematic where there is a limit to the rewinding length due to structural constraints of the strapping head, winder and drive arrangement or due to load compression constraints (e.g., a not readily compressible load).
In another type of winder, a rotating head is formed having a stationary element and a pivotal element that each define an outer surface around which the strap material is wound. A slot is defined between the elements through which the strap traverses.
The pivotal element is biased toward the stationary element, i.e., to close the slot, by a spring. The biased element must be xe2x80x9cpulledxe2x80x9d away from the stationary element in order to open the strap slot so that the strap can readily traverse through the slot. The pulling of the pivotal element is carried out by a large wrap spring positioned on a shaft at the rear of the winder.
Although this winder has been found to work well, there are a number of drawbacks. First, the elements are biased toward one another, which requires a relatively large force to open the elements to establish the strap path. Second this arrangement uses a complex cam and plate system to properly xe2x80x9ctimexe2x80x9d the winder operating modes, e.g., the feed and retraction operating modes and to maintain the strap slot open. It has been found that the complexity of the winder, in conjunction with the large wrap spring can require more maintenance than practicable given the operational requirements of the strapping machines, generally.
Accordingly, there exists a need for a winder for a strapping machine that uses the tension in the strapping material to maintain the winder closed (i.e., to secure the strap in the winder). Desirably, in such a winder, the winder nevertheless begins to close, effectively tensioning the strap, before the strap winds over itself, without the need for a high rate spring. Desirably, such a winder is effective over a range of strap gauges and can be used with highly compressible loads. More desirably, such a winder can also provide a high tension in the rewound strap.
A winder for use in a strapping machine of the type for positioning a strap material around an associated load and tensioning the strap material around the load includes a frame for supporting the load, a chute positioned on the frame for receiving the strap material and orienting the strap material around the load, a strap supply and a strapping head for extracting the strap from the supply, feeding the strap through the chute around the load, passing the strap from the chute around the load, retracting and tensioning the strap.
The winder is positioned at the strapping head. Briefly, the strapping head includes a single pair of rollers for both feeding and retracting the strap, and a winder for tensioning the strap around the load. The strapping machine also includes a weld head for welding the overlapping strap sections to one another. In a present arrangement, the strapping head and weld head are separate units.
In one embodiment, the winder includes a rotating head portion having a stationary element and a pivotal element. The stationary and pivotal elements each define an outer surface around which the strap material is wound and also define a slot therebetween for receiving the strap material. The elements each include a gripping portion at about respective ends opposingly facing one another.
The pivotal element is pivotal between an open position in which the gripping portions are spaced from one another and a closed position in which the gripping portions cooperate with one another to engage and secure the strap material therebetween. Preferably, the pivotal element is biased to the open position.
The winder includes a pivot assist assembly to assist moving the pivotal element gripper toward the stationary element gripper to enhance securing the strap between the grippers. In a present embodiment, the pivot assist assembly includes a pin extending from the pivotal element and a track portion stationary relative to the rotating head portion. The track has a generally circular shape and is configured for the pin to traverse around the track. The track has a first portion having a first diameter and a second diameter less than the first diameter. The differences in relative diameters can be at the outer diameters.
The winder rotates from a home position in which the pin is in the first portion of the track and the winder is in the open position to an other than home position in which the pin is in the second portion of the track such that the pin engages a wall of the track at the second diameter urging the winder toward the closed position.
In one embodiment of the winder, the pivot member is positioned at about an inlet of the winder, at a location upstream of the pivotal element gripping portion. In this embodiment, the pivot member is disposed at about the outer surface, e.g., at about a periphery, of the pivotal element. Alternately, the pivot member can be disposed intermediate the pivotal element gripping portion and a strap exit of the winder.
Preferably, the pivotal element gripping portion is mounted in the element for pivoting movement independent of the movement of the pivotal element. This permits a gripper pad to lie on the strap and conform to the strap path as it traverses through the winder.
A current winder includes an over-rotation plate to permit rotation of the winder in excess of 360 degrees. The plate is positioned between the winder and the strapping head frame.
These and other features and advantages of the present invention will be apparent from the following detailed description, in conjunction with the appended claims.